The proteoglycan 4 (prg4) gene encodes for highly glycosylated proteins termed megakaryocyte stimulating factor (MSF), lubricin, and superficial zone protein (SZP) (1)). Lubricin was first isolated from synovial fluid and demonstrated lubricating ability in vitro similar to synovial fluid at a cartilage-glass interface (2). Lubricin was later identified as a product of synovial fibroblasts (3) and also shown to possess boundary lubricating ability at a latex-glass interface by Jay et al. (3-9). O-linked β(1-3)Gal-GalNAc oligosaccharides within a large mucin like domain of 940 amino acids (10), encoded for by exon 6, were subsequently shown to mediate, in part, this boundary lubricating ability (8). SZP was first localized at the surface of explant cartilage from the superficial zone and isolated from conditioned medium (11). SZP also demonstrated lubricating ability at a cartilage-glass interface (12). These molecules are collectively referred to as PRG4. PRG4 was also shown to be present at the surface of synovium (58), tendon (13), and meniscus (14). In addition, PRG4 has been shown to contribute, both at physiological and pathophysiological concentrations, to the boundary lubrication of apposing articular cartilage surfaces (59).
The functional importance of prg4 was shown by mutations that cause the camptodactyly-arthropathy-coxa vara-pericarditis (CACP) disease syndrome in humans. CACP is manifest by camptodactyly, noninflammatory arthropathy, and hypertrophic synovitis, with coxa vara deformity, pericarditis, and pleural effusion (15). Also, in PRG4-null mice, cartilage deterioration and subsequent joint failure were observed (16). Therefore, PRG4 expression is a necessary component of healthy synovial joints.
PRG4 is a member of the mucin family, which are generally abundant on epithelial linings and provide many functions, including lubrication and protection from invading microorganisms (17). The functional properties of mucins are generally determined by specialized glycosylation patterns and their ability to form multimers through intermolecular disulfide bonds (18), both of which are altered in chronic diseases (e.g. cystic fibrosis, asthma) (7). Biochemical characterization of PRG4 isolated from synovial fluid (2, 19) showed molecular heterogeneity in O-glycosylation, which appears to influence lubricating properties (8) Recently, PRG4 from bovine synovial fluid has been shown to exist as disulfide-bonded dimers, in addition to the monomeric forms, as suggested by the conserved cysteine-rich domains at both N- and C-terminals, along with an unpaired cysteine at the C-terminal (20).
In tissues such as synovial joints, physicochemical modes of lubrication have been classified as fluid film or boundary. The operative lubrication modes depend on the normal and tangential forces on the articulating tissues, on the relative rate of tangential motion between these surfaces, and on the time history of both loading and motion. The friction coefficient, μ, provides a quantitative measure, and is defined as the ratio of tangential friction force to the normal force. One type of fluid-mediated lubrication mode is hydrostatic. At the onset of loading and typically for a prolonged duration, the interstitial fluid within cartilage becomes pressurized, due to the biphasic nature of the tissue; fluid may also be forced into the asperities between articular surfaces through a weeping mechanism. Pressurized interstitial fluid and trapped lubricant pools may therefore contribute significantly to the bearing of normal load with little resistance to shear force, facilitating a very low μ. Also, at the onset of loading and/or motion, squeeze film, hydrodynamic, and elastohydrodynamic types of fluid film lubrication occur, with pressurization, motion, and deformation acting to drive viscous lubricant from and/or through the gap between two surfaces in relative motion.
The relevant extent to which fluid pressure/film versus boundary lubrication occurs classically depends on a number of factors (31). When lubricant film can flow between the conforming sliding surfaces, which can deform elastically, elastohydrodynamic lubrication occurs. Pressure, surface roughness, and relative sliding velocity determine when full fluid lubrication begins to break down and the lubrication enters new regimes. As velocity decreases further, lubricant films adherent to the articulating surfaces begin to contribute and a mixed regime of lubrication occurs. If the velocity decreases even further and only an ultra-thin lubricant layer composed of a few molecules remain, boundary lubrication occurs. A boundary mode of lubrication is therefore indicated by a friction coefficient (ratio of the measured frictional force between two contacting surfaces in relative motion to the applied normal force) during steady sliding being invariant with factors that influence formation of a fluid film, such as relative sliding velocity and axial load (35). For articular cartilage, it has been concluded boundary lubrication is certain to occur, although complemented by fluid pressurization and other mechanisms (36-39).
In boundary lubrication, load is supported by surface-to-surface contact, and the associated frictional properties are determined by lubricant surface molecules. This mode has been proposed to be important because the opposing cartilage layers make contact over ˜10% of the total area, and this may be where most of the friction occurs (30). Furthermore, with increasing loading time and dissipation of hydrostatic pressure, lubricant-coated surfaces bear an increasingly higher portion of the load relative to pressurized fluid, and consequently, this mode can become increasingly dominant (31, 32). Boundary lubrication, in essence, mitigates stick-slip (31), and is therefore manifest as decreased resistance both to steady motion and the start-up of motion. The latter situation is relevant to load bearing articulating surfaces after prolonged compressive loading (e.g., sitting or standing in vivo) (33). Typical wear patterns of cartilage surfaces (34) also suggest that boundary lubrication of articular cartilage is critical to the protection and maintenance of the articular surface structure.
With increasing loading time and dissipation of hydrostatic pressure, lubricant-coated surfaces bear an increasingly higher portion of the load relative to pressurized fluid, and consequently, μ can become increasingly dominated by this mode of lubrication. A boundary mode of lubrication is indicated by values of μ during steady sliding being invariant with factors that influence formation of a fluid film, such as relative sliding velocity and axial load. Boundary lubrication, in essence, mitigates stickslip, and is therefore manifest as decreased resistance both to steady motion and the start-up of motion.
The accumulation of PRG4 within synovial fluid and at the articular surface, are likely key functional determinants of PRG4's boundary lubricating ability. Recently, it was demonstrated that a significant, threefold secretion of PRG4 resulted from the dynamic shear loading of cultured cartilage explants, as compared to free-swelling or statically compressed cultures (27). This PRG4 synthesis and secretion by chondrocytes could significantly contribute to the concentration of PRG4 within synovial fluid, in both homeostatic and pathological conditions where physiological regulators are present (23). Although the amount of PRG4 bound to the surface does not appear to correlate with secretion rates, previous studies suggest surface bound PRG4 can exchange with endogenous PRG4 in synovial fluid (25), especially under the influence of mechanical perturbation (26, 27). Clarification of the spatial and temporal aspects of PRG4 metabolism within the joint, particularly at the articular surface, would further the understanding of PRG4's contribution to the low-friction properties of articular cartilage, and possibly lead to treatments to prevent loss of this function (40, 41). More remains to be determined about the processing, and the potentially additional or alternative functions of various PRG4 molecules of different molecular weight (10, 27, 28, 61). Moreover, the combination of chemical and mechanical factors to stimulate PRG4 expression in chondrocytes near the articular surface may be useful for creating tissue engineered cartilage from isolated sub-populations (29) with a surface that is bioactive and functional in lubrication.
The precise mechanisms of boundary lubrication at biological interfaces are currently unknown. However, proteoglycan 4 (PRG4) may play a critical role as a boundary lubricant in articulating joints. This secreted glycoprotein is thought to protect cartilaginous surfaces against frictional forces, cell adhesion and protein deposition. Various native and recombinant lubricin proteins and isoforms have been isolated and characterized. For instance, U.S. Pat. Nos. 5,326,558; 6,433,142; 7,030,223, and 7,361,738 disclose a family of human megakaryocyte stimulating factors (MSFs) and pharmaceutical compositions containing one or more such MSFs for treating disease states or disorders, such as a deficiency of platelets. U.S. Pat. Nos. 6,960,562 and 6,743,774 also disclose a lubricating polypeptide, tribonectin, comprising a substantially pure fragments of MSF, and methods of lubricating joints or other tissues by administering tribonectin systemically or directly to tissues.